1. Field of the Invention
This invention relates to a variable wavelength semiconductor laser having a variably controllable oscillation frequency which can be utilized in the fields of optical information processing, optical communication and so forth.
2. Description of the Related Art
Since a variable wavelength semiconductor laser can generate laser light of a required wavelength readily, it is expected to be applied, as an optical frequency band utilizing optical device for wavelength multiplex communication or signal processing, to optical communication and optical information processing systems. Further, the variable wavelength semiconductor laser is useful also for optical measuring systems including scientific measurement.
Conventionally, as systems which realize a variable wavelength semiconductor laser, principally the two systems of
1. variable wavelength distribution Bragg reflecting mirror semiconductor lasers, and PA1 2. external resonator type semiconductor lasers, as that disclosed in J. Verdiell et al., SPIE, Vol. 2,690, pp. 286-295 and that disclosed in Okai, Applied Physics, Vol. 63, pp. 2-13 have been investigated.
FIG. 1 is a view showing a structure in principle of a representative variable wavelength distribution Bragg reflecting mirror semiconductor laser. The variable wavelength distribution Bragg reflecting mirror semiconductor laser is constructed such that it is electrically separated into the two regions of gain region 31 and passive distribution Bragg reflecting mirror region 32 in a laser resonator and current can be injected to the regions independently of each other. To gain region 31, current is injected through laser electrode 34, but to passive distribution Bragg reflecting mirror region 32, current is injected to wavelength control electrode 35.
Passive distribution Bragg reflecting mirror region 32 has a characteristic that the optical refractive index thereof with respect to the laser oscillation wavelength is varied by a plasma effect when current is injected thereto. An end face of passive distribution Bragg reflecting mirror region 32 on the gain region side is formed as an interface at which it contacts with air by cleavage and acts as a reflecting mirror having a reflection factor of approximately 30%. Meanwhile, on another end face of passive distribution Bragg reflecting mirror region 32 on the distribution Bragg reflecting mirror region side, nonreflective coating 33 of a dielectric multiple layer film is provided.
Since the effective resonator length of the laser resonator varies by a variation of the refractive index of passive distribution Bragg reflecting mirror region 32 and the Bragg wavelength varies simultaneously, the reflection characteristic of the distribution Bragg reflecting mirror varies. By effects of the variations of both of them, the wavelength of laser oscillation light 36 varies. It is known that, accordingly, the present laser can vary the wavelength of laser oscillation light 36 by adjusting the current injection amount to the distribution Bragg reflecting mirror region.
Meanwhile, FIG. 2 is a view showing a structure in principle of an external resonator type semiconductor laser. In the external resonator type semiconductor laser shown in FIG. 2, one end face of semiconductor laser 41 is formed as high reflection coating 47 while the other end face of semiconductor laser 41 is formed as nonreflective coating 46, and light generated in active region 45 of semiconductor laser 41 and emitted from the end face of semiconductor laser 41 on nonreflective coating 46 side is coupled to diffraction grating 44 by collimate lens 42 so that a laser resonator is formed from the high reflection coating 47 and diffraction grating 44.
Since diffraction grating 44 has a wavelength selective reflection characteristic in accordance with the incidence angle as well known in the art, the laser oscillation wavelength can be varied by rotating diffraction grating 44.
However, such variable wavelength semiconductor lasers as described above have the following problems.
The variable wavelength distribution Bragg reflecting mirror semiconductor laser shown in FIG. 1 has a problem in that, from a reason in principle that the variable wavelength band is narrow and effects of both of a variation of the effective resonator length of the laser resonator and a variation of the reflection characteristic of the distribution Bragg reflecting mirror by a variation of the refractive index of distribution Bragg reflecting mirror region 32 participate in the variation of the oscillation wavelength of the laser, a jump between longitudinal modes occurs upon wavelength variation and consequently the wavelength band which allows continuous variation is small.
In order to solve the problem just described, also a three-electrode type variable wavelength distribution Bragg reflecting mirror semiconductor laser has been attempted wherein a phase adjustment region is provided between the gain region and the distribution Bragg reflecting mirror region and current is supplied to the phase adjustment region to cancel the variation of the refractive index of distribution Bragg reflecting mirror region 32. Also in this instance, however, the continuously variable wavelength width remains within the order of several nm. Besides, since the structure is complicated, there is a problem in that it is difficult to produce the laser.
The external resonator type semiconductor laser shown in FIG. 2 has a problem in that, although the continuously variable wavelength width is large, since it is not a monolithic device, it is difficult to utilize in terms of the size and handling.